1. Field of the Invention
The present invention relates to a radar, in particular, an on-vehicle radar used for determining whether a detected target is a false image.
This application is based on Patent Application No. Hei 10-191260 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
A conventional on-vehicle radar used for preventing collisions of vehicles or the like is known, which is excellent in determination or judgment of an obstacle existing in the horizontal direction. For example, a radar using the amplitude comparison monopulse method is known (refer to Japanese Unexamined Patent Application, First Publication, Nos. Hei 9-68573 and 8-334557, and the like). In the operation of such a radar using the amplitude comparison monopulse method, (i) a radio wave is radiated via a plurality of antenna elements which are arranged such that portions of radiating patterns thereof overlap with each other, (ii) a signal reflected by a target is received by a plurality of antenna elements, (iii) the frequency of each received signal is converted into an intermediate frequency, (iv) the frequency-converted signal is converted into a digital signal, and (v) the converted digital signal is supplied to a signal processor so as to detect a target based on a frequency-analysis method such as the FFT (fast Fourier transform).
In the conventional radar, a false image of the target, generated by a side lobe of the beam received by the antenna, is determined with reference to the difference between the received power at the angular position of the main lobe and the received power at the angular position of the side lobe.
FIG. 8 is a diagram explaining a method for determining a false image based on the difference of values or amounts of power, performed in the above-described conventional radar.
In part (A) of the figure, curves a and b respectively indicate received power Pra and received power Prb of the signal received by two receiving antennas. These power energies belong to the main lobes. In part (B) of FIG. 8, curve c indicates the ratio (Pr .DELTA./Pr .SIGMA.) of the sum to the difference of the received power values. In the signal processor, threshold Prth is defined as shown in part (A) of FIG. 8, so that the angle calculation for detecting the angle (of the target) is not performed for signals having a level below the threshold.
Therefore, the angle calculation is not performed for the dotted-lined parts of curve c, corresponding to side-lobe angle ranges, so that no false image of the target is generated in those ranges. Here, in order to enable the angle calculation within an angle-measurement requesting range, threshold Prth must be smaller than the minimum received power Prmin in an angle-calculating (or measuring) range.
However, the above conventional radar has the following problem. The received power Pr is represented by the following formula (using the radar equation): ##EQU1## where Pt denotes the transmitted power, .lambda. denotes the wavelength of the transmitted signal, Gt denotes the transmitting gain, .sigma. denotes the effective reflection area of the target, Gr denotes the receiving gain, and R denotes the distance to the target.
In the above formula, Pt, Gt, Gr, and .lambda. have values peculiar to the radar. If a high-pass filter having a cut-off frequency of f.sup.4 (f is the intermediate frequency) is used in the receiving circuit (so as to perform the attenuation correction using "R"), then the received power Pr depends on the effective reflection area .sigma. of the target.
FIG. 9 is a diagram showing received power Pra and received power Prb, and the ratio Pr .DELTA./Pr .SIGMA. when the effective reflection area .sigma. of the target is large. As shown in part (A) in FIG. 9, the received power at the angular position corresponding to each side lobe exceeds the threshold Prth, and false images are generated in the corresponding angular ranges.
FIG. 10 is a diagram showing received power Pra and received power Prb, and the ratio Pr .DELTA./Pr .SIGMA. when the effective reflection area .sigma. of the target is small. As shown in part (A) in FIG. 10, the minimum received power Prmin within the angle-measurement requesting range is smaller than the threshold Prth; thus, the angle cannot be calculated within the relevant range, and the angle calculation cannot be performed in the angle-measurement requesting range.
Therefore, in order that the received power at the angular range of each side lobe is equal to or smaller than the threshold Prth with respect to the upper limit of the effective reflection area .sigma. of the target, and that the minimum received power Prmin within the angle-calculating range is equal to or larger than the threshold Prth with respect to the lower limit of the effective reflection area .sigma. of the target, an antenna having low-level side lobes is necessary. However, there is a limit to reduce the level of the side lobe, and an antenna having reduced-level side lobes is expensive.